The PDF4LHC Working Group Interim Recommendations (1101.0538v1)

Abstract: This note provides an interim summary of the current recommendations of the PDF4LHC working group for the use of parton distribution functions (PDFs) and of PDF uncertainties at the LHC, for cross section and cross section uncertainty calculations. It also contains a succinct user guide to the computation of PDF uncertainties and correlations using available PDF sets. A companion note (the PDF4LHC Working Group Interim Report) summarizes predictions for benchmark cross sections at the LHC (7 TeV) at NLO using modern PDFs currently available from 6 PDF fitting groups.

• The paper establishes unified interim recommendations for applying parton distribution functions in LHC cross-section calculations at NLO and NNLO.
• It details a conservative envelope method combining uncertainties from multiple PDF sets at NLO to benchmark prediction reliability.
• The guidance scales NNLO uncertainties based on NLO analyses, ensuring consistent and accurate theoretical predictions in collider physics.

Analysis of the PDF4LHC Working Group Interim Recommendations

The document titled "The PDF4LHC Working Group Interim Recommendations" presents a curated set of directives from the PDF4LHC working group for the utilization and analysis of parton distribution functions (PDFs) in the context of Large Hadron Collider (LHC) experiments. Parton distribution functions are critical in computing theoretical predictions for various cross sections at the LHC, and the uncertainty associated with these computations. This paper serves as a practical guideline to provide consistency and accuracy in cross-section predictions based on current datasets and methodologies.

Overview of the Recommendations

The interim recommendations consolidate the knowledge and insights from multiple PDF sets produced by various groups, namely CTEQ, MSTW, and NNPDF, among others, into a unified strategy that can be employed by experimental and theoretical physicists working with LHC data. The guidance comprehensively addresses both next-to-leading order (NLO) and next-to-next-to-leading order (NNLO) prescriptions for utilizing PDFs in cross-section calculations.

NLO vs. NNLO Considerations

1. NLO Prescriptions: For NLO calculations, the paper advises the usage of predictions from CTEQ6.6, MSTW2008, and NNPDF2.0. Key to this recommendation is the employment of a conservative envelope approach, which takes into account the combined uncertainties from the central values and $\alpha_s$ uncertainties across these PDF sets. The focus is to ensure that the uncertainties in predictions are reflective of inter-set variability, achieved by creating an envelope of predictions.
2. NNLO Prescriptions: Given the availability of fewer NNLO datasets, specifically MSTW2008, the recommendation extrapolates the uncertainties derived from NLO calculations to the NNLO space. The paper suggests scaling the NNLO uncertainties by a factor derived from the NLO analysis to conservatively estimate true uncertainty extents at NNLO. This approach recognizes the broader uncertainty estimated using the envelope method at NLO when applied singularly to the MSTW dataset in NNLO conditions.

Implications and Future Directions

The recommendations outlined possess both immediate practical and long-term theoretical implications. Practically, they assist in maintaining a consistent and realistic estimation of uncertainties in LHC cross-section data, facilitating cross-comparison and benchmarking across different experimental results. Theoretically, these prescriptions help delineate the bounds of precision that current PDF analyses can offer, pointing towards areas where further refinement or additional data integration could enhance predictive accuracy. As new datasets and PDF determinations emerge, the standardizations and practical steps provided here are expected to evolve, responding to the dynamics of collider physics and the continuous quest for precision.

The emphasis on using a PDF uncertainty prescription underscores ongoing discussions in the scientific community about the balance between pragmatic approximations and comprehensive precision. As LHC experiments continue, the working group's efforts to establish standardizations at both NLO and NNLO levels remain pivotal, promoting an overarching coherence in interpreting and applying theoretical predictions in high-energy physics research.

Conclusion

This document provides an essential step towards harmonizing the use of parton distribution functions in LHC data analysis. The interim recommendations from the PDF4LHC working group reflect a detailed synthesis of multiple PDF sets tailored to the unique challenges posed by LHC experiments. While addressing current experimental needs, the recommendations also set a precedent for analytic adaptiveness as new theoretical insights and empirical data become available, thereby paving the way for more refined and accurate particle physics research.